Although stannous fluoride has been used as a proven anti-caries agent for over twenty years, its mechanism of action is still not clearly understood. Only recently has the compound Sn3F3PO4 been identified as one of the main reaction products of in vitro experiments involving the treatment of enamel surfaces with 10% stannous fluoride solutions. It is possible that Sn3F3PO4 plays an important role in the anti-caries mechanism of stannous fluoride in vivo. This investigation will study the reaction of stannous fluoride with sound and carious dental enamel as it exists in the human mouth. Approximately 100 children will be enrolled in a clinical program. The children receive oral health care instructions, a SnF2 containing dentifrice, dental floss and the SnF2 gel is to be brushed on the teeth on a weekly basis. The shed exfoliated teeth will be collected for chemical and x-ray analyses. Surface reaction in the first 10 to 25 micron layer will be identified using X-ray powder diffraction methods. The products in the carious dentin will be analyzed similarly. The enamel fluoride biopsy technic will be used to remove the first two to three microns of enamel surface for chemical assay using atomic absorption, fluoride electrode, and other chemical methods. The reaction products of the hydroxyapatite-stannous fluoride system will be isolated in pure form. Acid solubility studies will be done on these compounds using conventional technics.